Responses of abdominal vascular capacitance in the anaesthetized dog to changes in carotid sinus pressure.

1. The abdominal circulation of anaesthetized dogs was vascularly isolated without opening the abdomen, by cutting or tying all structures immediately above the diaphragm and tying the proximal ends of the hind limbs. The region was perfused at constant flow through the aorta and drained at constant pressure from the inferior vena cava. 2. Vascular resistance responses were expressed as the changes in perfusion pressure and capacitance responses were determined by integrating changes in vena caval outflow. 3. Decreasing the pressure in the isolated carotid sinuses over the whole baroreceptor sensitivity range increased mean perfusion pressure from 91 to 149 mmHg (a 67% increase in resistance) and decreased mean capacitance by 111 ml. (5 ml. kg-1). 4. The range of carotid sinus pressures over which capacitance responses occurred was at a significantly higher level than the corresponding range for resistance responses. 5. Comparison of the reflex responses with the responses to direct stimulation of efferent sympathetic nerves shows that quantitatively similar responses of resistance and capacitance to those induced by a large step decrease in carotid pressure could be produced by stimulating maximally the efferent sympathetic nerves at 5 Hz. These results also suggest that at all levels of carotid sinus pressure there is no difference in the impulse traffic to resistance and capacitance vessels.